1. Field of the Invention
The present invention relates to a heating furnace used in plastic optical fiber drawing devices.
2. Description of the Prior Art
FIG. 1 is a cross-sectional view of a plastic optical fiber drawing device using a prior art heating furnace. The drawing device is provided with a heating furnace 3 which first heats and melts a base material 1 and draws it into a fiber. The device is also provided with a base-material feeding device 5 which supplies the base material 1 into the heating furnace 3. The device further comprises a coiler 9 which reels the plastic optical fiber 7 made from the base material 1. Inside the heating furnace 3, there is provided a single cylindrical radiant heater 11 for heating and melting the base material 1.
The heater 11 has a length L of 60 mm, when measured in the drawing direction of the plastic optical fiber 7. The heater 11 supplies heat to the base material 1. The amount of heat supplied is related to the time required for passing the base material 1 through the heater 11, i.e., to the length L of the heater 11 and the feeding speed for the base material 1 (drawing line-speed). The drawing line-speed is limited by the length L of the heater 11. For example, with a heater 11 having a length of 60 mm, the drawing line-speed is limited to 5 m/min. When the speed is above this value, the heat transfer to the base material 1 is slowed down and the base material 1 is drawn in a half-melt state, so that the fiber may be cut off.
Usually when drawing plastic base material 1, a line speed of 10 m/min is considered to be a feasible criterion. In the drawing devices using a known heating furnace 3, the amount of heat provided by the heater 11 to the base material 1 is not sufficient. As a result, a line speed of only 5 m/min can be obtained, which raises a feasibility problem.
A method for enhancing the line speed may include increasing the length L of the heater 11. However, when only the heater 11 is lengthened, temperature variations become greater along the longitudinal direction of the heater 11. Consequently, the melt zone (neck-down zone) of the base material 1 forms an unstable shape. The external diameter of the plastic optical fiber 7 thus becomes less accurate.
For instance, when length L of the heater 11 is doubled to 120 mm and the line speed attains 10 m/min, the temperature variations along the heater 11 increase from .+-.10.degree. C. to .+-.20.degree. C. Consequently, the variations in the outer diameter of the plastic optical fiber 7 increase from .+-.30 .mu.m to .+-.50 .mu.m.
In view of this problem, an object of the present invention is to provide a heating furnace used in a drawing device for a plastic optical fiber, by virtue of which the drawing line-speed can be improved, without deteriorating the dimension accuracy of the resulting optical fiber.